In order to estimate the HRI response to a cosmic X-ray source,
several performance characteristics of the combined HRI and
ROSAT telescope are needed.
In the sections that follow we provide estimates of minimum
source count rate detection and expected count rates from sources.
The assumptions that were used in providing these estimates are:

The effective area shown in
Fig. 5.11 (Figure 11.1 of Appendix F)
and listed in
Tab. 5.4 (Table 11.1 of Appendix F)
is the product of the XRT area and the HRI quantum efficiency.

The HRI point source detection cell size is chosen so that
50 percent of the source photons fall within the
detection cell.
Hence the cell size must be matched to the XRT point spread function,
the HRI detector resolution, and the aspect uncertainty (the latter
currently being the largest factor in this choice).

The HRI background rate is discussed above in section 3.4.
For the present purposes, the X-ray background was computed
from the spectrum shown in Figure 11.2 of Appendix F,
which is a compilation of Wisconsin and LLL measurements
[Williamson
et al.1974, Bunner et al.1972]
of the diffuse galactic background, and an extrapolation
of a power law fit to the extragalactic background.
In estimating the HRI count rate we integrate this spectrum
over the total effective area of the XRT/HRI.
Note that the UV/Ion Filter and electrostatic shield greatly
reduce the HRI sensitivity at low energies and that there is
a major dip in effective area at the carbon edge.
It should also be noted that the galactic component of the
X-ray background varies with the viewing direction.
We have taken a conservative estimate of the galactic
background in the sense that we use the high emission estimates
in calculating the HRI count rates.
In some cases the HRI background count rates due to galactic
emission may be up to a factor of 5 lower than the value
of 1.0 count s used here and section 11 of Appendix F.

None of these assumptions is absolutely correct, but they provide a
typical description of the XRT+HRI performance which may be useful
in preparing ROSAT observation proposals.